glib: add internal macro G_THREAD_LOCAL for support for thread local storage

_Thread_local is also C11, so possibly other compilers would also support
it.

However, since not *all* compilers support it, it can anyway only be
used as optimization and conditional asserts. As such, the current
detection based on __GNUC__ to only support gcc (and clang) is good
enough.

glib/glib-private.h

@@ -321,4 +321,10 @@ GLibPrivateVTable *glib__private__ (void);
 gboolean g_uint_equal (gconstpointer v1, gconstpointer v2);
 guint g_uint_hash (gconstpointer v);
 
+#if defined(__GNUC__)
+#define G_THREAD_LOCAL __thread
+#else
+#undef G_THREAD_LOCAL
+#endif
+
 #endif /* __GLIB_PRIVATE_H__ */